Described here is a multidisciplinary program which is aimed at elucidating the cellular mechanisms involved in the accumulation and remodeling of muscle connective tissue and the reduced range of movement that occur in aging and/or inactive individuals. The first hypothesis is that the amount and properties of the connective tissue in the muscle belly are influenced by the type of activity that the muscle is subjected to and that stretch is the important factor. The second hypothesis is that connective tissue remodeling is linked with adjustments in sarcomere number so that there is a reduced range of movement as well as an increase in stiffness. The questions asked include: can regular exercise delay the onset of muscle stiffness by altering the distribution and nature of the connective tissue?; also, which type of stretch (active, passive, dynamic) is the main factor in preventing muscle connective tissue accumulation and sarcomere loss? The form and amount of muscle connective tissue will be studied by quantitative transmission, EM, and light microscopic techniques. Changes in endomysial and perimysial connective tissue will be evaluated and the rates of synthesis of collagen and contractile proteins will be measured to shed light on the remodeling process. Changes in the functional length of muscle fibers will be determined by measuring sarcomere number and sarcomere length. The cellular mechanisms involved in the addition and removal of sarcomeres will be studied concurrently with those involved in connective tissue remodeling and these will be related to the physical and physiologic characteristics of the muscles studied. These will include measurements of muscle force, rate of force development, compliance and elasticity. Both the morphologic and physiologic changes will then be related to the functioning (force production, compliance and elasticity) of the aging musculoskeletal system. The data should be of considerable value to those involved in physical therapy, the management of elderly respiratory, orthopedic and neurologic patients as well as elderly patients in general.